EP1489774B1

EP1489774B1 - System and method to determine a bit error probability of received data within a cellular wireless network

Info

Publication number: EP1489774B1
Application number: EP04014002A
Authority: EP
Inventors: Li Fung Chang; Yongqian Wang; Huaiyu Zeng; Xiaoxin Qiu; Baoguo Yang
Original assignee: Broadcom Corp
Current assignee: Broadcom Corp
Priority date: 2003-06-16
Filing date: 2004-06-15
Publication date: 2009-09-16
Anticipated expiration: 2024-06-15
Also published as: EP1489774A3; US20040252647A1; US7352720B2; DE602004023154D1; EP1489774A2

Description

BACKGROUND

1. Technical Field

The present invention relates generally to cellular wireless communication systems, and more particularly to the determination of a bit error probability of radio frequency communications received by a wireless terminal within a cellular wireless communication system.

2. Related Art

Cellular wireless communication systems support wireless communication services in many populated areas of the world. While cellular wireless communication systems were initially constructed to service voice communications, they are now called upon to support data communications as well. The demand for data communication services has exploded with the acceptance and widespread use of the Internet. While data communications have historically been serviced via wired connections, cellular wireless users now demand that their wireless units also support data communications. Many wireless subscribers now expect to be able to "surf" the Internet, access their email, and perform other data communication activities using their cellular phones, wireless personal data assistants, wirelessly linked notebook computers, and/or other wireless devices. The demand for wireless communication system data communications will only increase with time. Thus, cellular wireless communication systems are currently being created/modified to service these burgeoning data communication demands.
Cellular wireless networks include a "network infrastructure" that wirelessly communicates with wireless terminals within a respective service coverage area. The network infrastructure typically includes a plurality of base stations dispersed throughout the service coverage area, each of which supports wireless communications within a respective cell (or set of sectors). The base stations couple to base station controllers (BSCs), with each BSC serving a plurality of base stations. Each BSC couples to a mobile switching center (MSC). Each BSC also typically directly or indirectly couples to the Internet.
In operation, each base station communicates with a plurality of wireless terminals operating in its cell/sectors. A BSC coupled to the base station routes voice communications between the MSC and a serving base station. The MSC routes voice communications to another MSC or to the PSTN. Typically, BSCs route data communications between a servicing base station and a packet data network that may include or couple to the Internet. Transmissions from base stations to wireless terminals are referred to as "forward link" transmissions while transmissions from wireless terminals to base stations are referred to as "reverse link" transmissions. The volume of data transmitted on the forward link typically exceeds the volume of data transmitted on the reverse link. Such is the case because data users typically issue commands to request data from data sources, e.g., web servers, and the web servers provide the data to the wireless terminals. The great number of wireless terminals communicating with a single base station forces the need to divide the forward and reverse link transmission times amongst the various wireless terminals.
Wireless links between base stations and their serviced wireless terminals typically operate according to one (or more) of a plurality of operating standards. These operating standards define the manner in which the wireless link may be allocated, setup, serviced and torn down. One popular cellular standard is the Global System for Mobile telecommunications (GSM) standard. The GSM standard, or simply GSM, is predominant in Europe and is in use around the globe. While GSM originally serviced only voice communications, it has been modified to also service data communications. GSM General Packet Radio Service (GPRS) operations and the Enhanced Data rates for GSM (or Global) Evolution (EDGE) operations coexist with GSM by sharing the channel bandwidth, slot structure, and slot timing of the GSM standard. GPRS operations and EDGE operations may also serve as migration paths for other standards as well, e.g., IS-136 and Pacific Digital Cellular (PDC).
The GSM standard specifies communications in a time divided format (in multiple channels). The GSM standard specifies a 4.615 ms frame that includes 8 slots of, each including eight slots of approximately 577 µs in duration. Each slot corresponds to a Radio Frequency (RF) burst. A normal RF burst, used to transmit information, typically includes a left side, a midamble, and a right side. The midamble typically contain a training sequence whose exact configuration depends on modulation format used. However, other types of RF bursts are known to those skilled in the art. Each set of four bursts on the forward link carry a partial link layer data block, a full link layer data block, or multiple link layer data blocks. Also included in these four bursts is control information intended for not only the wireless terminal for which the data block is intended but for other wireless terminals as well.
GPRS and EDGE include multiple coding/puncturing schemes and multiple modulation formats, e.g., Gaussian Minimum Shift Keying (GMSK) modulation or Eight Phase Shift Keying (8PSK) modulation. Particular coding/puncturing schemes and modulation formats used at any time depend upon the quality of a servicing forward link channel, e.g., Signal-to-Noise-Ratio (SNR) or Signal-to-Interference-Ratio (SIR) of the channel, Bit Error Rate of the channel, Block Error Rate of the channel, etc. As multiple modulation formats may be used for any RF burst, wireless communication systems need the ability to determine which coding scheme and modulation format will result in the successful receipt and demodulation of the information contained within the RF bursts. This decision may be further influenced by changing radio conditions and the desired quality level to be associated with the communications.
Link adaptation (LA) is a mechanism used to adapt the channel coding schemes and modulation formats to the changing radio link conditions. LA allows the network to command the handset to change to the modulation and coding scheme that is best for the current radio condition while providing a desired level of quality associated with the communications. To facilitate the network to do so, the handset reports a downlink quality report or quality measure to the network via the servicing base station.
Examples of link adaptation mechanisms are disclosed in US 2002/0186761 , US 6 539 205 , and EP 1 176 750 .
Key challenges in LA are the algorithm used in the network for link adaptation control, and the accuracy of the downlink quality reports that measure the changing radio conditions. In general, the actual channel quality of the changing radio conditions may be represented measures such as the Bit Error Rate (BER) or Block Error Rate (BLER). However, exact BER evaluation is often intractable or numerically cumbersome. Therefore, approximations of the channel quality are sought. Such approximations may be referred to as the Bit Error Probability (BEP). The quality reported to the network and calculated by the handset are the long-term average and standard deviation of the BEP. For the current RLC block, these values may be referred to as BEP_{block, n} and CV_BEP_{block, n} respectively. BEP_{block, n} is the current BEP value averaged over 4 radio bursts for a given RLC block and CV_BEP_{block, n} is the corresponding standard deviation.
There are several ways to the long-term average of the BEP. For example, the BEP can be derived based on: (1) signal-to-noise ratio (SNR); (2) re-encoding correctly decoded data; or (3) the training sequence. SNR-based BEP requires robust SNR-to-BEP mapping table that covers all types of propagation environments. SNR based approximations often overestimate system performance. This over estimation of system performance can result in optimistic BEPs being used to make LA decisions. LA decisions based upon optimistic BEP can result in lost communications between the wireless terminal and the servicing base station. Furthermore, extensive computer simulations are therefore needed to generate this mapping table.
RBER count provides a better measurement for the current link quality regardless of the radio propagation environments. Thus re-encoding based BEP can better reflect the link quality, however, this value is available only if the data block is decoded correctly. Training sequence based BEP calculation can be easily obtained but it does not provide enough samples (26 for GMSK, 78 for 8PSK) for BEP averaging. Therefore there is a need to determine for more accurate BEP in the LA process.
According to the invention, there are provided a method to determine a channel quality as defined by independent claim 1 and a wireless terminal as defined by independent claim 8.
Further advantageous features of the invention are defined by the dependent subclaims.
In order to overcome the shortcomings of prior devices, the present invention provides a system and method to determine or estimate the channel quality with measures such as the bit error probability (BEP) of a received radio frequency (RF) bursts within a data frame that substantially addresses the above identified needs. The present invention offers algorithms that provide better results by deriving the BEP based on both: (1) signal-to-noise ratio (SNR); and (2) re-encoding correctly decoded data. More specifically, one embodiment of the present invention provides a method of processing RF bursts within a wireless terminal in order to determine the channel quality. These RF bursts are part of a data frame that is received from a servicing base station by a wireless terminal in a cellular wireless communication system. The signal to noise ratio (SNR) of the RF bursts is determined and sequences of soft decisions from the RF bursts are extracted. These soft decisions may be protected by cyclical redundant coding (CRC) and convolutional coding. Typically, the SNR is derived from the training sequences within the RF bursts and the SNR maps to an estimated BEP based upon the modulation format of the radio block. Soft decisions within the RF bursts may be extracted from the data bits within the RF bursts. The sequences of soft decisions from the data bits decode to produce a data block. A determination is also made whether the decoding of the sequence of soft decisions was successful. When the sequence of soft decisions decoded favorably the data block is re-encoded to produce a sequence of re-encoded decisions. Comparing the sequence of re-encoded decisions to the sequence of soft decisions yields a re-encoded bit error (RBER). When the decoding is unsuccessful, the BEP is estimated only upon the estimated BEP derived from the SNR. However, when the decoding is successful, both the estimated BEP and RBER may be used to determine the BEP.
Another embodiment of the present invention provides a wireless terminal having a radio frequency (RF) front end, a base band processor communicatively coupled to the RF front end, and an optional enCOder/DECoder (CODEC) processing module which if present is communicatively coupled to the base band processor. The combination of the RF front end, base band processor, and the CODEC processing module (if present) operate to receive and process RF bursts from the servicing base station. The combination determines the SNR of the RF bursts and uses the SNR to determine an estimated BEP. Sequences of soft decisions, extracted from the RF bursts, decode to produce a data block. The combination then determines whether or not the sequence of soft decisions was decoded successfully to the data block. When the sequence of soft decisions decoded favorably, the data block is re-encoded to produce a sequence of re-encoded decisions. The sequence of soft decisions when compared to the sequence of re-encoded decisions produces a RBER. The combination determines the BEP based upon the estimated BEP derived from the SNR when the decoding is unsuccessful. However, when the decoding is successful, the combination uses the sum of the estimated BEP and RBER weighted by the block error rate (BLER) to determine the BEP to be reported to the servicing base station. The BLER provides an important indicator of link quality that may comprise or be derived from a bitmap indicating which segments of the RF transmissions were requested to be retransmitted.
The weighting factors may vary depending on how the soft decisions decoded. In one specific instance where the soft decisions decoded unfavorably, the estimated BEP may be taken as the maximum of the estimated BEP based on SNR and RBER threshold, where the RBER threshold is predetermined by the modulation format and coding scheme. Choosing the maximum of the estimated BEP based on both SNR and RBER threshold value when soft decisions decode unfavorably may limit the reporting of an overly optimistic channel quality measure based only on the SNR.
In summary, based on the current invention the downlink quality report first use an estimated BEP based on the SNR of the RF bursts, RBER threshold, and the RBER of the RF bursts as the quality indication of the current received RLC block. The downlink quality is then updated using the current quality report, i.e. current block's BEP together with the historical measures such as the mean BEP and standard deviation of the BEP of a RLC block (4 radio bursts) averaged over the reporting period and all assigned time slots per modulation type. The quality is reported to the network when it is requested through signaling control messages.
Other features and advantages of the present invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram illustrating a portion of a cellular wireless communication system that supports wireless terminals operating according to the present invention;
FIG. 2 is a block diagram functionally illustrating a wireless terminal constructed according to the present invention;
FIG. 3 is a block diagram illustrating in more detail the wireless terminal of FIG. 2, with particular emphasis on the digital processing components of the wireless terminal;
FIG. 4 is a block diagram illustrating the general structure of a GSM frame and the manner in which data blocks are carried by the GSM frame;
FIG. 5 is a block diagram illustrating the formation of down link transmissions;
FIG. 6 is a block diagram illustrating the recovery of a data block from a down link transmissions;
FIG. 7 is a flow chart illustrating operation of a wireless terminal in receiving and processing a RF burst; and
FIG. 8 is a flow chart illustrating operations to recover a data block;
FIGs. 9A and 9B are logic diagrams illustrating methods for operating a wireless terminal to determine a BEP of received bursts; and
FIG. 10 is a logic diagram illustrating an example of a method for operating a wireless terminal to determine a BEP of received bursts.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram illustrating a portion of a cellular wireless communication system 100 that supports wireless terminals operating according to the present invention. The cellular wireless communication system 100 includes a Mobile Switching Center (MSC) 101, Serving GPRS Support Node/Serving EDGE Support Node (SGSN/SESN) 102, base station controllers (BSCs) 152 and 154, and base stations 103, 104, 105, and 106. The SGSN/SESN 102 couples to the Internet 114 via a GPRS Gateway Support Node (GGSN) 112. A conventional voice terminal 121 couples to the PSTN 110. A Voice over Internet Protocol (VoIP) terminal 123 and a personal computer 125 couple to the Internet 114. The MSC 101 couples to the Public Switched Telephone Network (PSTN) 110.
Each of the base stations 103-106 services a cell/set of sectors within which it supports wireless communications. Wireless links that include both forward link components and reverse link components support wireless communications between the base stations and their serviced wireless terminals. These wireless links support digital data communications, VoIP communications, and other digital multimedia communications. The cellular wireless communication system 100 may also be backward compatible in supporting analog operations as well. The cellular wireless communication system 100 supports the Global System for Mobile telecommunications (GSM) standard and also the Enhanced Data rates for GSM (or Global) Evolution (EDGE) extension thereof. The cellular wireless communication system 100 may also support the GSM General Packet Radio Service (GPRS) extension to GSM. However, the present invention is also applicable to other standards as well, e.g., TDMA standards, CDMA standards, etc. In general, the teachings of the present invention apply to digital communications that apply dynamic link adaptation (LA) of the Modulation and Coding schemes (MCSs) utilized for communications between wireless terminals and servicing base stations.
Wireless terminals 116, 118, 120, 122, 124, 126, 128, and 130 couple to the cellular wireless communication system 100 via wireless links with the base stations 103-106. As illustrated, wireless terminals may include cellular telephones 116 and 118, laptop computers 120 and 122, desktop computers 124 and 126, and data terminals 128 and 130. However, the wireless system supports communications with other types of wireless terminals as known to those skilled in the art as well. As is generally known, devices such as laptop computers 120 and 122, desktop computers 124 and 126, data terminals 128 and 130, and cellular telephones 116 and 118, are enabled to "surf" the Internet 114, transmit and receive data communications such as email, transmit and receive files, and to perform other data operations. Many of these data operations have significant download data-rate (forward link) requirements while the upload data-rate (reverse link) requirements are not as severe. Some or all of the wireless terminals 116-130 are therefore enabled to support the EDGE operating standard. These wireless terminals 116-130 also support the GSM standard and may support the GPRS standard. Wireless terminals 116-130 support the LA decision making process by determining the bit error probability (BEP) of received radio frequency (RF) communications received from by base stations 103-106 and reporting this BEP to the wireless communication system 100. The wireless communication system 100 then uses the provided BEP information to select an appropriate MCS.
Wireless terminals 116-130 support the pipelined processing of received RF bursts in slots of a GSM frame so that a plurality of slots in each GSM frame are allocated for forward link transmissions to a single wireless terminal. In one embodiment, a number of slots of a GSM frame are allocated for forward link transmissions to a wireless terminal such that the wireless terminal must receive and process a number of RF bursts, e.g., 2, 3, 4, or more RF bursts, in each GSM frame. The wireless terminal is able to process the RF bursts contained in these slots and still service reverse link transmissions and the other processing requirements of the wireless terminal.
FIG. 2 is a block diagram functionally illustrating a wireless terminal 200 constructed according to the present invention. The wireless terminal 200 of FIG. 2 includes an RF transceiver 202, digital processing components 204, and various other components contained within a case. The digital processing components 204 includes two main functional components, a physical layer processing, speech COder/DECoder (CODEC), and baseband CODEC functional block 206 and a protocol processing, man-machine interface functional block 208. A Digital Signal Processor (DSP) is the major component of the physical layer processing, speech COder/DECoder (CODEC), and baseband CODEC functional block 206 while a microprocessor, e.g., Reduced Instruction Set Computing (RISC) processor, is the major component of the protocol processing, man-machine interface functional block 208. The DSP may also be referred to as a Radio Interface Processor (RIP) while the RISC processor may be referred to as a system processor. However, these naming conventions are not to be taken as limiting the functions of these components.
The RF transceiver 202 couples to an antenna 203, to the digital processing components 204, and also to a battery 224 that powers all components of the wireless terminal 200. The physical layer processing, speech COder/DECoder (CODEC), and baseband CODEC functional block 206 couples to the protocol processing, man-machine interface functional block 208 and to a coupled microphone 226 and speaker 228. The protocol processing, man-machine interface functional block 208 couples to a Personal Computing/Data Terminal Equipment interface 210, a keypad 212, a Subscriber Identification Module (SIM) port 213, a camera 214, a flash RAM 216, an SRAM 218, a LCD 220, and LED(s) 222. The camera 214 and LCD 220 may support either/both still pictures and moving pictures. Thus, the wireless terminal 200 of FIG. 2 supports video services as well as audio services via the cellular network.
FIG. 3 is a block diagram illustrating in more detail the wireless terminal of FIG. 2, with particular emphasis on the digital processing components of the wireless terminal. The digital processing components 204 include a system processor 302, a baseband processor 304, and a plurality of supporting components. The supporting components include an external memory interface 306, MMI drivers and I/F 308, a video I/F 310, an audio I/F 312, a voice band CODEC 314, auxiliary functions 316, a modulator/demodulator 322, ROM 324, RAM 326 and a plurality of processing modules. In some embodiments, the modulator/demodulator 322 is not a separate structural component with these functions being performed internal to the baseband processor 304.
The processing modules are also referred to herein as accelerators, co-processors, processing modules, or otherwise, and include auxiliary functions 316, an equalizer module 318, an enCOder/DECoder (CODEC) processing module 320, and an Incremental Redundancy (IR) processing module 328. The interconnections of FIG. 3 are one example of a manner in which these components may be interconnected. Other embodiments support additional/alternate couplings. Such coupling may be direct, indirect, and/or may be via one or more intermediary components.
RAM and ROM service both the system processor 302 and the baseband processor 304. Both the system processor 302 and the baseband processor 304 may couple to shared RAM 326 and ROM 324, couple to separate RAM, coupled to separate ROM, couple to multiple RAM blocks, some shared, some not shared, or may be served in a differing manner by the memory. In one particular embodiment, the system processor 302 and the baseband processor 304 coupled to respective separate RAMs and ROMs and also couple to a shared RAM that services control and data transfers between the devices. The processing modules 316, 318, 320, 322, and 328 may coupled as illustrated in FIG. 3 but may also coupled in other manners in differing embodiments.
The system processor 302 services at least a portion of a serviced protocol stack, e.g., GSM/GPRS/EDGE protocol stack. In particular the system processor 302 services Layer 1 (L1) operations 330, a portion of Incremental Redundancy (IR) GSM protocol stack operations 332 (referred to as "IR control process"), Medium Access Control (MAC) operations 334, and Radio Link Control (RLC) operations 336. The baseband processor 304 in combination with the modulator/demodulator 322, RF transceiver, equalizer module 318, and/or encoder/decoder module 320 service the Physical Layer (PHY) operations performed by the digital processing components 204. The baseband processor 304 may also services a portion of the GSM/GPRS/EDGE protocol stack.
Still referring to FIG. 3, the baseband processor 304 controls the interaction of the baseband processor 304 and equalizer module 318. As will be described further, the baseband processor 304 is responsible for causing the equalizer module 318 and the CODEC processing module 320 to process received RF bursts that reside within slots of a GSM frame. In the particular embodiment of FIGs. 2 and 3, with single RF front end 202, wireless terminal 200 may receive and process RF bursts in up to four slots of each GSM frame, i.e., be assigned four slots for forward link transmissions in any particular GSM frame. In another embodiment in which the wireless terminal 200 includes more than one RF front end, the wireless terminal 200 may be assigned more than four slots in each sub-frame of the GSM frame. In this case, required transmit operations would be performed using a second RF front end while a first RF front end would perform the receive operations. When the forward link transmissions and the reverse link transmissions occupy different channels with sufficient frequency separation, and the wireless terminal otherwise supports full duplex operations, the wireless terminal could receive and transmit at the same time.
The combination of the RF front end 202, and base band processor 204, which may include an optional CODEC processing module, receive RF communications from the servicing base station. In one embodiment the RF front end 202 and base band processor 204 determine the SNR of the RF bursts. Then, the SNR is used to determine an estimated BEP. Concurrently or serially, base band processor 204 to produce a data block decodes sequences of soft decisions, extracted from the RF bursts. The sequence of soft decisions may decode successfully into the data block as indicated by error correction coding results. These soft decisions may be protected by cyclical redundant coding (CRC) and convolutional coding. Re-encoding of properly decoded data blocks produces a sequence of re-encoded decisions which when compared to the sequence of soft decisions produces a Re-encoded Bit Error (RBER). The BEP reported to the servicing base station is based upon the estimated BEP derived from the SNR and the RBER. When the decoding is unsuccessful, the BEP may be based upon more heavily or solely the estimated BEP provided by the SNR. Similarly, when the decoding is successful, the BEP may be based upon more heavily or solely the RBER or BLER. The BLER is often considered as giving a more objective quality measurement than the BEP or RBER. This allows the BEP to more accurately reflect actual channel conditions. Thus, LA decisions can more effectively select an appropriate MCS based upon existing channel conditions.
FIG. 4 is a block diagram illustrating the general structure of a GSM frame and the manner in which data blocks are carried by the GSM frame. The GSM frame is 4.615 ms in duration, including guard periods, and each of which includes eight slots, slots 0 through 7. Each slot is approximately 577 µs in duration, includes a left side, a midamble, and a right side. The left side and right side of a normal RF burst of the time slot carry data while the midamble is a training sequence.
The RF bursts of four time slots of the GPRS block carry a segmented RLC block, a complete RLC block, or two RLC blocks, depending upon a supported Modulation and Coding Scheme (MCS) mode. For example, data block A is carried in slot 0 of sub-frame 1, slot 0 of sub-frame 2, slot 0 of sub-frame 3, and slot 0 of sub-frame 3. Data block A may carry a segmented RLC block, an RLC block, or two RLC blocks. Likewise, data block B is carried in slot 1 of sub-frame 1, slot 1 of sub-frame 2, slot 1 of sub-frame 3, and slot 1 of sub-frame 3. The MCS mode or CS mode of each set of slots, i.e., slot n of each sub-frame, for the GSM frame is consistent for the GSM frame. Further, the MCS mode or CS mode of differing sets of slots of the GSM frame, e.g., slot 0 of each sub-frame vs. any of slots 1-7 of each sub-frame, may differ. This ability allows LA to be implemented. As will be described further with reference to FIG. 5, the wireless terminal 200 may be assigned multiple slots for forward link transmissions that must be received and processed by the wireless terminal 200.
FIG. 5 depicts the various stages associated with mapping data into RF bursts. A Data Block Header and Data are initially unencoded. The block coding operations perform the outer coding for the data block and support error detection/correction for data block. The outer coding operations typically employ a cyclic redundancy check (CRC) or a Fire Code. The outer coding operations are illustrated to add tail bits and/or a Block Code Sequence (BCS), which is/are appended to the Data. After block coding has supplemented the Data with redundancy bits for error detection, calculation of additional redundancy for error correction to correct the transmissions caused by the radio channels. The internal error correction or coding scheme of GSM is based on convolutional codes.
Some coded bits generated by the convolutional encoder are punctured prior to transmission. Puncturing increases the rate of the convolutional code and reduces the redundancy per data block transmitted. Puncturing additionally lowers the bandwidth requirements such that the convolutional encoded signal fits into the available channel bit stream. The convolutional encoded punctured bits are passed to an interleaver, which shuffles various bit streams and segments the interleaved bit streams into the 4 bursts shown.
Each RF burst has a left side, a midamble, and a right side. The left side and right side contain data. The midamble consists of predefined, known bit patterns, the training sequences, which are used for channel estimation to optimize reception with an equalizer and for synchronization. With the help of these training sequences, the equalizer eliminates or reduces the intersymbol interferences, which can be caused by propagation time differences of multipath propagation. A number of training sequences are defined for normal RF bursts in the GSM standard. However, the exact configuration of the training sequences may depend on the modulation format used. Each set of four bursts typically utilizes the same modulation format. By analyzing the training sequence one can determine the modulation format.
FIG. 6 is a block diagram depicting the various stages associated with recovering a data block from RF bursts. Four RF bursts making up a data block are received and processed. Once all four RF bursts have been received, the RF bursts are combined to form an encoded data block. The encoded data block is then depunctured (if required), decoded according to an inner decoding scheme, and then decoded according to an outer decoding scheme. For MCS1-4, the decoded data block includes the data block header and the data, for MCS5-9, data block and header block are coded separately. Successful decoding may be signaled by appropriate tailbits appended to the data following convolutional decoding (error correction coding).
FIG. 7 is a flow charts illustrating operation of a wireless terminal 200 in receiving and processing RF bursts. The operations illustrated correspond to a single RF burst in a corresponding slot of GSM frame. The RF front end 202, the baseband processor 304, and the equalizer module 318 illustrated in FIG. 3 perform these operations. These operations are generally called out as being performed by one of these components. However, the split of processing duties among these various components may differ without departing from the scope of the present invention.
A single processing device or a plurality of processing devices operably coupled to memory performs the processing duties. Such a processing device may be a microprocessor, micro-controller, digital signal processor, microcomputer, central processing unit, field programmable gate array, programmable logic device, state machine, logic circuitry, analog circuitry, digital circuitry, and/or any device that manipulates signals (analog and/or digital) based on operational instructions. The memory may be a single memory device or a plurality of memory devices. Such a memory device may be a read-only memory, random access memory, volatile memory, non-volatile memory, static memory, dynamic memory, flash memory, cache memory, and/or any device that stores digital information. Note that when the processing duties are implemented via a state machine, analog circuitry, digital circuitry, and/or logic circuitry, the memory storing the corresponding operational instructions may be embedded within, or external to, the circuitry comprising the state machine, analog circuitry, digital circuitry, and/or logic circuitry. The processing duties include the execution of operational instructions corresponding to at least some of the steps and/or functions illustrated in FIGs. 6-10.
Referring particularly to FIG. 7, operation commences with the RF front end 202 receiving an RF burst in a corresponding slot of a GSM frame (step 702). The RF front end 202 then converts the RF burst to a baseband signal (step 704). Upon completion of the conversion, the RF front end 202 stores the converted baseband signal. When needed the baseband processor samples the converted baseband signal from the RF front end. Thus, as referred to in FIG. 7, the RF front end 202 performs steps 702-704.
Operation continues with the baseband processor 304 receiving the baseband signal (step 708). In a typical operation, the RF front end 202, the baseband processor 304, or modulator/demodulator 322 samples the analog baseband signal to digitize the baseband signal. After receipt of the baseband signal (in a digitized format), the baseband processor 304 performs detection of a modulation format of the baseband signal (step 710). This detection of the modulation format determines the modulation format of the corresponding baseband signal. Proper determination of the modulation format is necessary in order to properly estimate the channel quality from the SNR of the channel. In one particular embodiment according to the GSM standard, the modulation format will be either Gaussian Minimum Shift Keying (GMSK) modulation or Eight Phase Shift Keying (8PSK) modulation. The baseband processor 304 makes the determination (step 712) and appropriately processes the RF bursts based upon the detected modulation format.
The baseband processor performs pre-equalization processing of the RF bursts in step 712. For GMSK modulation, this processing involves de-rotation and frequency correction; burst power estimation; timing, channel, noise, and signal-to-noise ratio (SNR) estimation; automatic gain control (AGC) loop calculations; soft decision scaling factor determination; and matched filtering operations on the baseband signal. For 8PSK modulation, pre-equalization processing of the RF bursts involves de-rotation and frequency correction; burst power estimation; timing, channel, noise, and SNR estimations; AGC loop calculations; Decision Feedback Equalizer (DFE) coefficients calculations; and soft decision scaling factors for the baseband signal. The SNR estimation from the pre-equalization processing operations may be used later to determine the estimated BEP. Determination of the estimated BEP will be discussed further in FIGs. 9-11. These pre-equalization processing operations produce a processed baseband signal. Upon completion of these pre-equalization processing operations, the baseband processor 304 issues a command to the equalizer module 318.
The equalizer module 318, upon receiving the command, prepares to equalize the processed baseband signal based upon the modulation format, e.g., GMSK modulation or 8PSK modulation in step 714. The equalizer module 318 receives the processed baseband signal, settings, and/or parameters from the baseband processor 304 and equalizes the processed baseband signal.
After equalization, the equalizer module 318 then issues an interrupt to the baseband processor 304 indicating that the equalizer operations are complete for the RF bursts. The baseband processor 304 then receives the soft decisions from the equalizer module 318. Next, the baseband processor 304 performs "post-equalization processing" as shown in step 716. This may involve determining an average phase of the left and right sides based upon the soft decisions received from the equalizer module 318 and frequency estimation and tracking based upon the soft decisions received from the equalizer module 318.
The sequences of soft decisions are decoded in step 718. One particular method of decoding the soft decisions is further detailed in FIG. 8. The decoded soft decisions may be used to produce a RBER. Failure to properly decode the soft decisions may be assigned a RBER threshold based on the modulation format and coding scheme. This process of producing an RBER will be described in further detail in association with the description of FIGs. 9-11 and following. With the estimated BEP and/or RBER, baseband processor 304 or system processor 302 produce a BEP in step 720, which in turn is reported to the servicing base station in step 722. While the operations of FIG. 7 are indicated to be performed by particular components of the wireless terminal, such segmentation of operations could be performed by differing components. For example, the baseband processor 304 or system processor 302 in other embodiments could perform the equalization operations. Further, the baseband processor 304 or the system processor 302 in other embodiments could also perform decoding operations.
FIG. 8 is a flow chart illustrating operations to decode a data block according to an embodiment of the present invention. Operations commence with receiving and processing an RF bursts (front-end processing of RF bursts) in step 802 and as described with reference to steps 702-716 of FIG. 7. After receiving the four RF bursts that complete an EDGE or GPRS data block, as determined at step 804, operation proceeds to step 806.
A header of the data block identifies the coding scheme and puncturing pattern of the data block. For example, the coding scheme may be any one of the MCS-1 through MCS-9 coding schemes, each of which may include multiple puncturing patterns. Operation uses the training sequence of each RF burst, located within the midamble of the RF burst, to identify the modulation format of the RF bursts.
Data recovery begins in step 806 where, if necessary, the data block is decrypted. The data block is then de-interleaved (step 808) according to a particular format of the data block, e.g.. MCS-1 through MCS-9. The data block is then de-punctured (step 810). At step 812, the de-interleaved and de-punctured data block is decoded. Decoding operations may include combining previously received copies of the data block with the current copy of the data block. Data bits of the decoded data block are then processed further (step 814). Properly decoded data blocks can be re-encoded to produce a sequence of re-encoded decisions that when compared to the sequence of soft decisions result in the RBER. The RBER may provide a more accurate indication of the performance of the selected MCS than that provided by the estimated BEP, which is based on the SNR.
FIG. 9A is a logic diagram illustrating a method for operating a wireless terminal to determine a BEP of a received bursts according to an exemplary method. The method commences with receiving the RF bursts in step 902. Next, the signal-to-noise (SNR) of the RF bursts is determined in step 904. The determination of the SNR in step 904 is typically completed as part of the pre-equalization processing of step 712. One example utilizes the extracted training sequence of a RF burst to produce a burst SNR. The block SNR averaging over 4 burst SNRs is mapped to an estimated BEP based on the modulation format. Continuing with step 906, a sequence of soft decisions is extracted from the payload of each RF burst. This sequence may correspond to the training sequence or data, wherein the greater number of samples available within the data portion would yield more accurate results than those derived from the smaller sample set of the training sequence. An attempt to decode the sequence of soft decisions extracted from data bits of the 4 RF bursts is made in step 908. When the attempt to decode the plurality of soft decisions is unsuccessful (as determined at decision point 910), the BEP of the RF bursts is determined based upon the SNR (estimated BEP) in step 912. When the sequence of soft decisions decodes successful at decision point 910, the data bits are re-encoded to produce a sequence re-encoded decisions at step 914. Then, the sequence of re-encoded decisions is compared to the sequence of soft-decisions in step 916. This comparison results in a RBER upon which the current BEP may be based (step 918). The BEP may be updated at servicing base station in step 920. This update may also include the historical performance of the current BEP in determining the BEP update to the servicing base station by including the mean and/or standard deviation of the historical or previous BEPs.
According to the invention, as illustrated in FIG. 9B, determines the estimated BEP in step 912 with the SNR value of step 904 based on the modulation format. Additionally, this embodiment assigns a threshold value to the RBER based on the modulation format and coding scheme when the attempt to decode the plurality of soft decisions is unsuccessful (as determined at decision point 910). This allows the current BEP of the RF bursts to be determined based upon a weighted sum of the SNR (estimated BEP of step 912) and RBER threshold when unsuccessful decoding occurs. This combination may reduce the BEP update to the servicing base station when a BEP update based only on SNR would prove overly optimistic. Similarly, when the sequence of soft decisions decodes successfully at decision point 910, the BEP of the RF bursts may be determined based upon both the SNR (estimated BEP of step 912) and RBER of the data bits, wherein different weighting values or coefficients are assigned to the SNR and RBER. These weighting values or coefficients may be based on whether or not the sequence of soft decisions decodes successfully. Alternatively, these weighting values may be based on the BLER. For example, the weighting values may weigh the SNR more heavily when the sequence of soft decisions decodes unsuccessfully. Alternatively, the weighting values may weigh the RBER more heavily when the sequence of soft decisions decodes successfully. Further, a comparison between the SNR and assigned threshold value of the RBER when the sequence of soft decisions decodes unsuccessfully may examine the relative magnitude of the estimated BEP and RBER and limit the BEP to be the lesser of the two.
When the sequence of soft decisions decodes successful at decision point 910, the data bits are re-encoded to produce a sequence re-encoded decisions at step 914. Then, the sequence of re-encoded decisions is compared to the sequence of soft-decisions in step 916. This comparison results in a RBER upon which the current BEP may be based (step 918). The BEP may be updated at servicing base station in step 920. This update may also include the historical performance of the current BEP in determining the BEP update to the servicing base station by including the mean and/or standard deviation of the historical or previous BEPs.
FIG. 10 is a logic diagram illustrating overall control flow for operating a wireless terminal to determine a BEP of received data block according to the present invention. A sequence of soft information of a RLC block extracted from payload of 4 RF bursts is received (step 1002). The sequence carries data and header. Upon receipt of the sequence, an attempt is made to decode the header (step 1004). Decoding the header allows the coding scheme to be readily identified. This information is coupled with knowledge to the modulation format to determine the MCS of the received sequence. If the decode is not successful (as determined at step 1006), no BEP update is performed (step 1008) and operation returns to step 1002 wherein another sequence is awaited. If the header decode is successful (as determined at step 1006), operation proceeds to step 1010 where the wireless terminal determines whether the data block carried in the bursts is intended for the wireless terminal. If the data block is not intended for the wireless terminal, the operation proceeds to step 1008 and no BEP update/calculation occurs. This prevents unnecessary BEP calculations and potential LA decisions being based on communications not intended for the wireless terminal.
If the data block carried in the sequence does belong to the wireless terminal (as determined at step 1010), the wireless terminal next determines whether the Block Sequence Number (BSN) of the data block is within a receiving window under consideration (step 1012). If not, the BEP is updated based upon the SNR of the block (at step 1014) and operation proceeds from step 1014 to step 1002 where another sequence is awaited. If the BSN is inside the receiving window (as determined at step 1012) the wireless terminal then attempts to decode the received data block (step 1018).
If the decode attempt of step 1020 is successful, as determined at step 1018, the decoded data is re-encoded in step 1022 to produce a RBER based upon the re-encoded data as was previously described in reference to steps 914 and 916 of FIGs. 9A and 9B, and the current BEP is based upon the RBER count in step 1024. If the decoding attempt is not successful (as determined at decision point 1020), operation proceeds to step 1026 where the current BEP is based upon the maximum of the estimated BEP based on SNR or pre-defined RBER threshold based on the modulation format as was previously described in reference to step 912 of FIGs. 9A and 9B. From both steps 1022 and 1026 operation proceed to step 1024 where the current BEP is determined. In step 1028, the BEP is updated and operations then return to step 1002. The BEP update may further include the historical performance of the BEP by including the long-term mean and/or standard deviation of the BEP.
In the operation of FIG. 10, SNR is typically calculated with the training sequence of the bursts to map to an estimated BEP and will be used for BEP calculation whenever the RBER is not available. Generally, the SNR provides a reasonable link quality measure for channels free of inter-symbol interference. For channels with inter-symbol interference, SNR alone is not sufficient to measure the quality of the link. Instead, RBER count is a better quality measure. When data decoding fails, the RBER cannot be obtained. In this case, SNR-based BEP calculation will be used with the additional fact that data decoding has failed. This information can help better quantify the channel quality in combination with the SNR. When data decoding fails, the number of errors in the received block has to exceed some threshold. Therefore, the RBER can be assumed to reach this RBER threshold. This threshold, for a given MCS mode, is related to the error correction capability of the mode and can be obtained via measurements or simulations. The threshold needs to be carefully selected and tested.
As one of average skill in the art will appreciate, the term "substantially" or "approximately", as may be used herein, provides an industry-accepted tolerance to its corresponding term. Such an industry-accepted tolerance ranges from less than one percent to twenty percent and corresponds to, but is not limited to, component values, integrated circuit process variations, temperature variations, rise and fall times, and/or thermal noise. As one of average skill in the art will further appreciate, the term "operably coupled", as may be used herein, includes direct coupling and indirect coupling via another component, element, circuit, or module where, for indirect coupling, the intervening component, element, circuit, or module does not modify the information of a signal but may adjust its current level, voltage level, and/or power level. As one of average skill in the art will also appreciate, inferred coupling (i.e., where one element is coupled to another element by inference) includes direct and indirect coupling between two elements in the same manner as "operably coupled". As one of average skill in the art will further appreciate, the term "compares favorably", as may be used herein, indicates that a comparison between two or more elements, items, signals, etc., provides a desired relationship. For example, when the desired relationship is that signal 1 has a greater magnitude than signal 2, a favorable comparison may be achieved when the magnitude of signal 1 is greater than that of signal 2 or when the magnitude of signal 2 is less than that of signal 1.
The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiment was chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

A method to determine a channel quality, as represented by a bit error probability, BEP, of received Radio Frequency, RF, bursts within a data frame received from a servicing base station by a wireless terminal in a cellular wireless communication system, the method comprising:
receiving (902) the RF bursts of the data frame from the servicing base station;

determining (904) a signal to noise ratio, SNR, of the RF bursts;

extracting (906) a sequence of soft decisions from the payload of the RF bursts;

decoding (908) the sequence of soft decisions to produce a data block;

determining (910) whether decoding of the sequence of soft decisions was successful;

re-encoding (914) the data block to produce a sequence of re-encoded decisions when decoding of the sequence of soft decisions was successful;

comparing (916) the sequence of soft decisions to the sequence of re-encoded decisions to produce a re-encoded bit error rate, RBER, when decoding of the sequence of soft decisions was successful;

determining (918) the current block BEP based on:
when decoding of the sequence of soft decisions was unsuccessful, a maximum of an estimated BEP derived from the SNR, and a RBER threshold predetermined by a modulation format and a coding scheme ;

when decoding of the sequence of soft decisions was successful, a weighted sum of the estimated BEP and the RBER.
The method of Claim 1, further comprising determining the BEP based on historical BEP data.
The method of Claim 1, wherein the sequences of soft decisions are extracted from data bits within the RF bursts.
The method of Claim 1, wherein cyclical redundant coding protects the sequence of soft decisions.
The method of Claim 1, wherein the SNR is derived from training sequences within the RF bursts.
The method of Claim 1, wherein SNR maps to the estimated BEP based on a modulation format of the RF bursts and wherein the RBER threshold is based on the modulation format and a coding scheme of the RF bursts.
The method of Claim 5, wherein the modulation format of the RF bursts is GMSK or 8PSK.
A wireless terminal that comprises:
a Radio Frequency, RF, front end (202); and

a baseband processor (304) communicatively coupled to the RF front end (202);

wherein, the RF front end (202) and the baseband processor (304) are operable to:
receive an RF burst of a data frame from a servicing base station;

determine a signal to noise ratio, SNR, of the RF burst;

extract a sequence of soft decisions from the RF burst;

decode the sequence of soft decisions to produce a data block;

re-encode the data block to produce a sequence of re-encoded decisions when decoding of the sequence of soft decisions was successful; and

compare the sequence of soft decisions to the sequence of re-encoded decisions to produce a re-encoded bit error rate, RBER, when decoding of the sequence of soft decisions was successful;

determine a current block bit error probability, BEP, based on:
when decoding of the sequence of soft decisions was unsuccessful, a maximum of an estimated BEP derived from the SNR, and a RBER threshold predetermined by a modulation format and a coding scheme;

when decoding of the sequence of soft decisions was successful, a weighted sum of the estimated BEP and the RBER; and

update a long-term BEP and report the long-term BEP to the servicing base station upon request.
The wireless terminal of claim 8 further comprising an enCOder/DECoder, CODEC, processing module (320) communicatively coupled to the baseband processor (304). of the sequence of soft decisions was